The Aes Sedai, both collectively and individually, never seem to lack for funds. Where do they get the money?

Individual sisters are given a yearly allowance of one thousand crowns in gold, which is comparable to the income of a very succesful merchant or a minor noble. More can requested, but it is subject to the review of the Hall. [NS 13: Business in the City] Additionally, newly raised sisters are usually made a gift of modest (by Aes Sedai standards) clothing and furnishings for their quarters. [NS 12: Entering Home] Sisters like Moiraine who come from a noble background might have their own estates they can draw income from as well.

The Tower itself obtains income from several sources:

• The Tower maintains it's own bank, which is considered the oldest and safest in Randland. It can be assumed that it collects fees and interest in the course of conducting it's usual business. [NS 13]
• The Tower used to collect a monthly tribute from the nations, however this practice has fallen into disuse with only the Borderlands still continuing the tradition. [TGS 18: A Message in Haste]

Other ideas:

• Income from property owned by the Tower and/or the Ajahs
• Tar Valon straddles the crossroads of some of the major trade routes on the continent. The Tower likely collects docking fees, tariffs, etc.
• Gifts from various nobles/merchants for services (e.g. Healing) and as bribes
• The Tower probably taxes the residents of Tar Valon and the surrounding villages.